As mentioned earlier, utilizing the extension of the S-K coding
scheme in an additive Gaussian noise MAC, the sum-capacity (REF ) can be achieved [1]}. Here, using our results on the gradient of mutual information in the Gaussian MAC with feedback, we show how S-K coding
scheme increases the capacity of MAC with feedback in comparison with the capacity of MAC without feedback. Applying S-K coding
scheme for the channel model (REF ) and (REF ), considering Remark REF , and
following similar steps as in the proof of Corollary REF , it can be easily shown that,
\(\frac{{\partial }}{{\partial \gamma }}I_{S-K}\left( {M_{\left( 1 \right)} ,M_{\left( 2 \right)} ;Y^n } \right) =\frac{1}{2}{mmse_{S-K}}\left( \gamma \right) + \vartheta _{S-K} \left( \gamma \right) + \zeta _{S-K} \left( \gamma \right)\) 
